as was evident by the effects produced in using the Telescope. The Speculum was therefore taken off, and the following device, technically termed "burning," was had recourse to. The entire brass work was imbedded in sand and covered to the depth of three inches. Down to the centre was bored a hole, one inch and a-half in diameter, and into this from a height of ten inches was poured a stream of melted brass, similar in composition to that of which the bed was made, until forty-five pounds had overflowed the surface of the sand. This operation was repeated in thirtyfour different places on the combined edges of the portions of the brass bed. The constant heat kept up by the stream on these parts fused the metal, which contracted on cooling, and united them perfectly in these situations. The other surface of the bed was attempted to be managed in the same way, but the process failed here; some of the brass was therefore chiselled out of the necessary points and more melted in, which contracted when cool and fully answered the desired end.

When it was found that the joinings were now secure, the Speculum metal and bed were again soldered together;

this, however, was no easy matter, and gave rise at first to much annoyance and loss of time. It was attempted to solder them by tinning the brass bed, and placing the Speculum on this with resin interposed, then raising the temperature and using slight pressure when the tin and resin were fused. However, the latter becoming decomposed during the elevation of the temperature effectually prevented any union taking place. Sal Ammoniac was tried with no better success. The process was therefore modified, and the junction of the metals was accomplished in the following manner.

The pieces of the Speculum metal being ground by the hand to fit the brass bed, they were scraped on one side with hard steel chisels, no spot being left untouched and made perfectly dry; the brass was also scraped and tinned, and all the resin washed off with turpentine, and afterwards with soap and water. The Speculum was then laid on the brass, and both were placed in an oven with an iron bottom and supported on bricks. When the tin was fused, which was effected in eight hours by a gradually raised temperature, melted resin and tin were poured in between the plates,

B

and they were moved gently backwards and forwards. The fire was withdrawn when the tin commenced acting on the Speculum, and the pieces being settled into the places where they were to remain-th of an inch apart all was allowed to cool gradually, and in five days it was ready to be ground. The casting of a Speculum, and its preservation from breaking when cooling, has always been found a matter of the greatest difficulty. The compound is so exceedingly brittle that any sudden change of temperature is likely to destroy it; and it requires to be so free from pores that unless great care is taken to give exit to all the air contained in the melted metal before setting, it is rendered useless. In casting the pieces of the compound Speculum, Lord Rosse first tried the plan recommended by Mr. Edwards, making the mould in sand, but the Speculum was almost always full of flaws, and generally flew in pieces when setting; on attempting to put the parts together again, it was found that they no longer fitted in their places. It was evident from this circumstance that they must have undergone some great strain; and it was judged that this had been produced by the unequal cooling of the different portions -the edges becoming solid first left the centre fluid, and this being, when about to set, unable to contract, was strained when no longer ductile. The failures in this way were so frequent that another plan was tried, which was this:-A number of equidistant plates of iron were placed in a crucible with the fluid Speculum metal, under the idea that it would be divided when solid, into an equal number of Specula. But this plan had no success at all; the plates were always full of flaws.

The next attempt was much more encouraging; a block of the Speculum metal was cut into plates by a circular sawing machine, the blades of which were of soft iron, kept constantly wetted with emery and water. This process gave pieces that were entirely free from flaws; however, the texture was not perfectly uniform throughout, for near the circumference the arrangement of the particles was not just the same as in the other parts, so that it was feared that a good Speculum would not be formed. It has, however, never been sufficiently well tried, another method having been discovered before this was fairly tested. This process which has deservedly stamped Lord Rosse's name with celebrity, and reduced the casting of Specula to a certainty, is perhaps the most deserving of praise of all his Lordship's works. The simplicity of the contrivance, probably at first sight, makes it appear a result of no great wonder: but like the plan pursued by Columbus to make the egg stand, it is only easy when known. It has been stated that the chief conditions so hard to be attained in the manufacture of Specula are, first-certainty of not breaking in cooling; and secondly-a freedom from pores, if it has been so fortunate as to escape the first. Now we saw that the mould of sand did not fulfil the first intention, from the unequal contraction the metal suffered when cooling; on this account a mould was made of cast-iron, in the hope that its high conducting power would lower the temperature of the bottom portions of the fluid metal first, and abstracting the heat much more quickly than it could escape from the surface, it would cool the metal gradually towards the top, and in this manner, prevent any part of it being strained. In order to keep the under surface of the mould at a low temperature, a jet of cold water was made to play on it